1. Field of the Invention
The invention relates to a magnetic resonance imaging apparatus which includes a coil system which is substantially rotationally symmetrically arranged about a symmetry axis, and a power supply device for feeding the coil system with current in order to generate a steady, homogeneous magnetic field in a measuring space of the apparatus, which coil system consists of a first and a second substantially rotationally symmetrical coil sub-system, said two coil sub-systems being situated in a substantially rotationally symmetrical space which has a U-shaped cross-section in a plane containing the symmetry axis, the open side of said U-shaped cross-section facing the symmetry axis and the rotationally symmetrical space of U-shaped cross-section containing a first sub-space parallel to the symmetry axis and a second sub-space transverse the symmetry axis, the first coil sub-system being situated exclusively in the first sub-space whereas the second coil sub-system is situated exclusively in the second sub-space, the first coil sub-system conducting a first current component whereas the second coil sub-system conducts a second current component, the two current components having mutually opposed directions.
2. Description of Related Art
A magnetic resonance imaging apparatus for medical purposes, also referred to as an MRI apparatus, is arranged to form images of cross-sections of a body. To this end, in such an apparatus a strong, steady, homogeneous magnetic field is generated in a volume intended for imaging (the imaging volume). On this homogeneous field a gradient field is superposed in order to indicate the location of the cross-section to be imaged. The atoms in the tissue present in the imaging volume are then excited by means of an RF field; the radiation released upon relaxation of the excited atoms is used to form an image of the cross-section indicated by the gradient field. The steady, homogeneous field (also referred to as the main field) is generated by means of a coil system (superconducting or not). Together with the associated envelope, the coil system is shaped as a short tube which contains the imaging volume. The diameter of this tube is determined by the dimensions of the patients to be examined so that it has a given minimum value, for example of the order of magnitude of 90 cm. Generally speaking, the aim is to minimize the length of this tube relative to the diameter in order to mitigate feelings of claustrophobia for the patients to be examined and to keep the area to be examined in the imaging volume as accessible as possible for the attending staff.
The cited European patent application describes an MRI apparatus having a comparatively short coil system, so a short tubular patient space. Notably FIG. 5 of the cited patent application shows a rotationally symmetrical coil system which is composed of a number of coils (denoted by the references 19, 20 and 21) which form part of two imaginary coil sub-systems. Thus, the coils 19 and 21 shown form part of a first imaginary coil sub-system and the coils 20 shown form part of a second imaginary coil sub-system.
These two coil sub-systems are arranged in a rotationally symmetrical space about a symmetry axis (denoted by the reference 7). This space can be considered to be the envelope of the coils 19-21 such that it has a U-shaped cross-section in a plane through the symmetry axis, the open side of the U facing the symmetry axis. The first sub-space is then formed by the part of the U which extends parallel to the symmetry axis whereas the second sub-space is formed by the two limbs of the U which, therefore, extend perpendicularly to the symmetry axis. The first coil sub-system (i.e. the sub-system consisting of the coils 19 and 21) is then situated in the first sub-space and the second coil sub-system (i.e. the sub-system consisting of the coils 20) is situated in the second sub-space. A power supply device applies a current through the coils; this current may be considered to consist of two components, i.e. a first current component which flows through the first coil sub-system and a second current component which flows through the second coil sub-system. The two current components thus defined have mutually opposed directions in the known MRI apparatus as appears from the "Table 3" included in the cited document and from the associated description. In the known coil system a further coil (denoted by the reference numeral 18) is situated in the space between the two limbs of the U and also contributes to the generating of the main field.
Even though this known coil assembly realizes a short tubular patient space, it does not provide shielding of the main field. If such shielding were required, the known system would have to be provided with additional means for shielding, for example active shielding coils which counteract the magnetic stray field outside the imaging volume as well as possible. Furthermore, in the tubular patient space of the known system a space has to be reserved for gradient coils and for coils for forming the RF field, so that the accessibility of the region to be examined is restricted to an undesirable extent.
Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicant's invention of the invention subsequently claimed.